Don’t assume timersub and gettimeofday

Don’t assume timersub and gettimeofday

POSIX does not specify timersub, and marks gettimeofday as
obsolescent.  Avoid porting problems by using timespec.h
functions instead.
* src/editfns.c: Include systime.h instead of sys/time.h.
(EXTRA_CONTEXT_FIELDS): Replace start and max_secs with
time_limit.  All uses changed.  This removes the need to call
gettimeofday or timersub.
* src/term.c (timeval_to_Time): Remove.  Replace all uses with ...
(current_Time): ... this new function, removing the need to
call gettimeofday.